The existing code was using:
  a[b][c[d]]
where a[b] would always exist and c[d] would always exist.
But the nested index would not alway exist, particularly when a DigitalOcean item disapeared.

This changes that nested index to a `get` and properly handles None results.

This bug was present in two spots in this file.

The `dopy` library function all_images includes a 'global' filter
by default.  This was preventing private images from showing up
in --images.

The problem also made this script fail immediately for people
who had droplets previously created with private images.

* Always refresh droplet information (versus getting it from cache) for
  --list, --host, --droplets, and --all.   All the DigitalOcean calls are
  pretty fast and without this change, inventory was often stale/incorrect.
  For example, previously if you destroyed a droplet, the inventory
  script would still find it and attempt to act on it.

* Added --force-cache option

* Added some option shortcuts -d, -r

* Added more value sanitization

Intended to complement the recently added digital_ocean module.